COS 55-5 - Seedling recruitment trends indicate climate-induced shifts in the structure and dynamics of Pacific Northwest forests

Abstract: As climate continues to change, seedlings are likely to recruit under substantially different conditions than those under which current canopy trees established, providing opportunities for delayed shifts in the structure and dynamics of forest communities. Natural climatic stress gradients offer complex ecological environments over which to evaluate the potential direct and indirect effects of climate change on seedling recruitment. In this study, we use three years of seed rain data and four years of seedling germination data for two abundant western conifer species to investigate variation in the limiting effects of climate and aboveground competition on fecundity and seedling germination along a natural climatic stress gradient. We monitored seeds and seedlings of Douglas-fir and western hemlock across five closed-canopy, naturally regenerated conifer forest stands extending from the Pacific Coast to the crest of the Cascade Mountain Range in western Oregon, USA. Summer temperature and precipitation represent the major climatic limitations on tree growth across all elevations and define the climatic stress gradient. We applied hierarchical Bayesian models with site-varying coefficients to identify interannual trends in fecundity and seedling germination, and to quantify the contributions of seasonal and annual climate and aboveground competition to these trends. Our results demonstrate that the factors limiting the early stages of tree recruitment vary in accordance with climate stress. For western hemlock, a smaller-seeded, shade-tolerant species, recruitment was most strongly limited by the effects of understory vegetation on seedling germination, an effect which strengthened with climatic stress. For Douglas-fir, a larger-seeded, moderately shade-intolerant species, early recruitment was primarily limited by the effects of summer drought stress on seed production. Temperature exerted a stronger influence than light competition on all early recruitment stages across most of the stress gradient. For both species, these limiting effects contributed to near recruitment failure at the most climatically stressful locations. These findings add to observations of radial growth declines, increased biotic disturbance, and drought-induced mortality in adult trees at the most climatically stressful location. Our results highlight the overwhelming influence of climate on the success of early recruitment and indicate a strong potential for recruitment declines under increasing climatic stress.